1. Technical Field
The disclosure relates to signal communications, and, more specifically but not exclusively, to correcting phase errors in signal communications.
2. Description of the Related Art
This section introduces aspects that may be helpful to facilitating a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
In conventional coherent communications systems, the phase of the receiver's local oscillator might not match the phase of the transmitter's local oscillator. If this phase mismatch is not corrected at the receiver, then the receiver might not properly recover data transmitted from the transmitter to the receiver. One method of correcting this phase mismatch is to adjust the receiver's local oscillator such that the phase of the receiver's local oscillator closely or exactly matches that of the transmitter's local oscillator. Such phase match can be accomplished using a phase-locked loop. However, in some systems such as optical systems, phase-locked loops can be complicated, expensive, and unreliable.
Rather than using a phase-locked loop to correct the phase mismatch, digital signal processing can be used downstream of the receiver's local oscillator to estimate and correct phase errors of complex symbols carried in the received signal without adjusting the phase of the receiver's local oscillator. Such digital processing may include both (i) coarse phase-error estimation and (ii) fine phase-error estimation.
In general, coarse phase-error estimation may be performed by estimating the phase errors of pilot symbols that are intermittently embedded in the received signal by the transmitter. The expected phases of the pilot symbols are known a priori by the receiver, and the receiver uses this knowledge to estimate the phase errors of the pilot symbols. The estimated phase errors of the pilot symbols are further used to estimate phase errors of complex data symbols transmitted between the pilot symbols. Techniques for performing the pilot-aided coarse phase-error estimation are known and therefore are not described herein.
The use of pilot symbols in coarse phase-error estimation does not account for phase wandering that exists in-between the intermittent pilot symbols (i.e., among the data symbols). Therefore, fine phase-error estimation may be performed to improve the phase-error estimates of the data symbols between the pilot symbols. One method for performing fine phase-error estimation, known as a blind phase search, is described in Pfau et al., “Hardware-Efficient Coherent Digital Receiver Concept with Feedforward Carrier Recovery for M-QAM Constellations,” Journal of Lightwave Technology, Vol. 27, No. 8, pp. 989-999 (Apr. 15, 2009), the teachings of all of which are herein incorporated by reference in their entirety. Another method for performing fine phase-error estimation, known as the Viterbi and Viterbi algorithm is described in Savory, “Digital Coherent Optical Receivers: Algorithms and Subsystems,” IEEE Journal of Selected Topics in Quantum Electronics, Vol. 16, No. 5, pp. 1164-1179 (September-October 2010), the teachings of all of which are incorporated herein by reference in their entirety.
After compensating for coarse and fine phase errors, there may be some relatively small residual phase errors in the compensated complex symbols that cause relatively insignificant degradation in the recovered signal quality. In addition to these residual phase errors, there is a possibility of a cycle slip, which may be more detrimental than the small residual phase errors. A cycle slip occurs when a stream of adjacent complex symbols are in error such that each complex symbol in the stream falls into a quadrant (in the case of QPSK and QAM) of the modulation constellation that is different from the quadrant used to generate the complex symbol. If the stream of errors is relatively long, then the errors might not be correctable using forward-error correction which usually has a limited burst-error correction capability.